Wikipedia
MicroXwin is a replacement graphics display framework for Linux operating systems. MicroXwin’s custom Xlib library sends graphics commands directly to the graphics driver, which handles all the drawing. As a client-server architecture is not used, it reduces the load on CPU and memory, since the processor does not need to perform a number of operations on the exchange of messages between the server and the client. It copies the data from user space to the frame buffer, and then on the screen.
MicroXwin module requires only 500KB of memory, whereas the standard X-system takes almost 30MB.
High performance is because MicroXwin has been designed as a kernel module. However, due to this, the collapse of MicroXwin will also crash the kernel. Furthermore, there is no network transparency unlike X11.